The efficiency of solar heat collectors is affected by the amount of convection, conduction and radiation losses associated with its construction. It has been found that heat losses due to convection and conduction may be materially reduced by evacuating the air space within the solar collector about the absorber or collector plate. However, in the case of standard flat plate solor heat collectors having standard sized flat plate absorbers, it has not been possible to evacuate such collectors and provide a single solar window over the same which is supported solely about edge portions thereof, due to the atmospheric load which is exerted thereon upon evacuation of the collector. That is, with a standard atmospheric pressure of about 15 lbs./sq. in., an evacuated relatively shallow solar collector structure will have an atmospheric pressure of approximately one ton per square foot on the collector window.
Realizing that it was impossible to subject the standard flat window of a flat plate heat collector to such forces without catastrophic failure, the prior art devices such as shown in U.S. Pat. No. 3,929,122 and No. 3,974,823 utilized two solar windows spaced apart from one another to form a dead air space between the ambient atmosphere and the interior of the solar collector containing the absorber plate. Although the dead air space provided a degree of insulation, convection and conduction heat losses of significant magnitude were still experienced.
In order to provide the desired vacuum within the solar collector so as to minimize convection and conduction heat losses from the absorber plate through the collector window to the ambient atmosphere, tubular solar collectors were utilized as shown in U.S. Pat. No. 3,227,153. The use of the tubular construction, which is strong in compression, permitted the evacuation of the solar collector and thereby materially reduced heat losses due to convection and conduction. However, the diameter of the solar collector was of course limited to practical aspects which accordingly limited the area of the flat plate collector or absorber member retained therein. Thus, in order to obtain the same surface area as was obtainable with a standard flat plate collector of a relatively large shallow structure, it was necessary to provide a multiplicity of such tubular collectors.
Other attempts have been made in supporting expansive flat solor windows in evacuated flat plate collectors, such as utilizing support posts as shown in U.S. Pat. No. 3,995,615 and longitudinally extending partition walls as shown in U.S. Pat. No. 4,038,965. Although the support posts of the former patent permit utilization of a standard flat plate collector, each of the support posts in fact functions as a conductor to conduct heat to the window and thus to the ambient atmosphere. In the case of the latter patent, the plurality of longitudinal partitions necessitate the utilization of a plurality of small collector plates similar to that used in the evacuated tubular collector, and again such partitions function as conductors or heat sinks to the solar window with the resultant loss of efficiency.
U.S. Pat. No. 3,986,491 discloses the use of a sheet of transparent or translucent corrugated plastic positioned above and across a metallic heat collecting surface having hills and valleys, with the corrugation as far as possible focusing the rays of the sun on one side of the flaring hills in the morning and focusing the solar rays on the opposite side of the hills during the afternoon. However, the solar collector is not evacuated, as an air space is provided within the collector between the transparent or translucent plastic solar window and the metallic collector plate.
Accordingly, the present invention has overcome the problems of providing an evacuated flat plate collector with a substantially continuous solar window which is supported solely about peripheral portions and which will withstand the atmospheric forces exerted thereon by utilizing a contoured solor window having a sinusoidal longitudinal cross section and a parabolic transverse cross section.